VMC 211: General Veterinary Microbiology (1+1)

METHODS USED IN CLASSIFICATION OF BACTERIA

• Three methods are used for classifying bacteria.

The intuitive method

• The microbiologist thoroughly familiar with the properties of the organism he is  studying for many years decides that it represents a species or genera.
• In this the characteristics of organism that seem important to one person may not be considered important by another person.

Numerical taxonomy

• Many characteristics of an organism are studied (usually 100 – 200) and each characteristic is given equal weight. Then the percent similarity of each strain to every other strain is calculated. The formula is

%S = NS/NS + ND

• NS is the number of characteristics that are the same (positive or negative) for both the strains. ND is the number of characteristics that are different. Sometimes a rigorous method also is used to calculate the percent similarity where the NS is taken as the number of positive characteristics that are same for both the strains. What the organisms can do is important than what they cannot do.
• The strains having a high %S are placed into groups. The groups having a high %S to each other are in turn placed into larger groups and so on. This method of classification has great practical utility and it is also relatively unbiased. It gives high degree of stability and predictability.

Genetic relatedness

• This is the most objective of all the classification methods. The DNA of the different organisms with respect to the sequence of their component nucleotides is compared.
• The methodologies used are
  • DNA homology experiments
    • The double stranded DNA from two organisms are heated to separate the strands and then mixed. This leads to the formation of heteroduplexes ie one strand from one organism will base pair with a strand from another organism.
    • If the organisms are not closely related then heteroduplexes will not be formed. This method is useful at species leve classification.
  • Ribosomal RNA homology
    • The nucleotide sequence of ribosomal RNA is highly conserved.
    • The degree of similarity can therefore be used to measure the relationship between organisms a level higher than the species.